An example of a photographing lens used with imaging elements such as CCDs is a photographing lens used for capturing video such as with monitoring cameras, which are primarily used to capture video. Since the pixel count in the imaging element is relatively low, the lens itself does not need to have high-quality optical properties.
The image quality of imaging elements used in conventional monitoring cameras and video cameras is generally inferior compared to the image quality of cameras using silver halide film. With recent advances in imaging element technology, however, the image quality of conventional monitoring cameras and video cameras has approached the image quality of silver halide film cameras. With recent increases in compactness and density now possible in imaging elements, there is a need for a photographing lens that provides high performance and that is also compact, thin, and inexpensive.
In photographing lenses used in devices such as portable telephones and portable information terminals (PDAs), the lens design is very compact and thin and is limited to roughly one or two lenses. However, the lenses are designed for relatively low pixel densities of approximately 100,000–350,000 pixels. Thus, the resulting images are not satisfactory.
Also, with imaging elements such as CCDs, a microlens can be disposed on the surface of the imaging element in order to use light efficiently. As a result, vignetting takes place if the angle of incoming light is too large, thereby preventing light from entering the imaging element. In order to overcome this problem, these conventional photographing lenses have generally provided an adequate distance between the exit pupil and the image plane, thus improving telecentricity by keeping the angle at which light enters the imaging element, i.e., the exit angle, small (for example, see Japanese Laid-Open Patent Document Number Hei 2000-171697, Japanese Laid-Open Patent Document Number Hei 2001-133684, Japanese Laid-Open Patent Document Number Hei 2002-98888, Japanese Laid-Open Patent Document Number Hei 2002-162561, Japanese Laid-Open Patent Document Number Hei 05-40220, Japanese Laid-Open Patent Document Number Hei 05-157962).
With recent significant technical developments in imaging elements, there has been a demand for more compact, thinner, more inexpensive photographing lenses with higher resolutions. With conventional imaging lenses, however, the need to improve telecentricity has resulted in relatively longer total lens lengths, thus preventing these lenses from achieving a thin design.
While the restrictions imposed by the exit angle of light rays have prevented the conventional imaging lenses from being thinner (i.e., having a shorter total lens system length), innovations in microlenses have made it possible to use exit angles of up to around 20°. As a result, there is a demand for a thinner photographing lens suited for an imaging element that uses this type of microlens.